MRAM is a nonvolatile memory technology that uses magnetization to represent stored data. Generally, MRAM includes a plurality of magnetic cells in an array. Each cell typically represents one bit of data. Included in the cells are magnetic elements. A magnetic element may include two ferromagnetic “plates” (or layers upon a semiconductor substrate) each of which has a magnetization direction (or orientation of magnetic moments) associated with it. The two ferromagnetic plates are separated by a thin non-magnetic layer.
More specifically, a MRAM element is often based on a magnetic tunnel junction (MTJ) element. An MTJ element includes at least three basic layers: a “free layer,” a tunneling barrier layer, and a “fixed layer.” The free layer and the fixed layer are ferromagnetic layers; the tunneling barrier layer is a thin insulator layer located between the free layer and the fixed layer. The magnetization direction of the free layer is free to rotate, but is constrained by the physical size of the layer to point in either of two directions; the magnetization of the fixed layer is fixed in a particular direction. A bit is written to the MTJ element by orienting the magnetization direction of the free layer in one of the two directions. Depending upon the orientations of the magnetic moments of the free layer and the fixed layer, the resistance of the MTJ element will change. Thus, the bit may be read by determining the resistance of the MTJ element. When the magnetization direction of the free layer and the fixed layer are parallel and the magnetic moments have the same polarity, the resistance of the MTJ element is low. Typically, this is designated a “0.” When the magnetization direction of the free layer and the fixed layer are anti-parallel and the magnetic moments have the opposite polarity, the resistance of the MTJ is high. Typically, this is designated a “1.”
Spin torque transfer (STT) (also known as spin transfer switching or spin-transfer effect) is one technique for writing to MTJ memory elements. STT is based upon the idea that when a spin-polarized current (most of the electrons of the current have spins aligned in the same direction) is applied to a free ferromagnetic layer, the electrons may transfer their spin angular momentum to the free layer to switch the direction of magnetization of the free layer. The advantages of using STT for writing to magnetic elements include smaller bit size and lower writing current requirement. However, in STT the switch current required to switch the magnetization direction of the MTJ element from parallel to anti-parallel is 20-50% larger than that required to switch from anti-parallel to parallel. Furthermore, in a conventional STT MTJ element the larger parallel-to-anti-parallel switching current is limited by a “source degeneration” or the so called “source-site loading” effect. This source degeneration effect constrains the amount of current flowing through the MTJ element and may prevent the MTJ element from switching the magnetization direction from anti-parallel to parallel reliably. Accordingly, it is desirable to have a STT MTJ element that is not limited by the source degeneration effect to ensure reliable switching of the magnetization direction of the MTJ element from parallel to anti-parallel.